ON SPECIFIC THERAPEUTICS. 57 



unaltered for any number of inoculations. But 

 the results are very different if one tries to 

 inoculate with the diseased products occurring 

 in pigeons. For although the inoculations can 

 also be carried out in pigeons for any number of 

 times, yet in such series the virulence for fowls 

 has completely disappeared. You see, there- 

 fore, that we are here dealing with a process 

 closely related to the modifications which the 

 variola virus suffers in the organism of the cow. 

 The only difference exists in the fact that in the 

 one case the virulence for the original host is 

 only reduced, whilst in the case of bird-pox it is 

 completely destroyed. In my view this can be 

 explained only in one way, for it is obviously 

 impossible to suppose that by one passage 

 through the pigeon's organism the parasites in 

 question should obtain a new set of receptors 

 capable of seizing upon antagonistic bodies 

 present in the normal fowl's serum. It is far 

 more natural to suppose that we have to do 

 here with a form of atrepsy. It is safe to 

 assume that the chemical composition of fowls' 

 and pigeons' tissues is not identical, and that 

 therefore the parasites, in their passage through 

 the pigeon, must assimilate substances different 

 to those assimilated in their passage through 



